Aromatic bromine compounds have proven themselves well in numerous synthetic resins as flame retardants. Examples of such flame retardants are: penta- or decabromodiphenyl ether, octabromodiphenyl, the dibromopropyl ethers of tribromophenol or of tetrabromo-bis(hydroxyphenyl)propane, hexabromobenzene, and the like.
In the use of such aromatic bromine compounds in thermoplastic synthetic resins, processable only at high temperatures, high requirements are imposed on the thermal stability of such aromatic bromine compounds; in particular, splitting off of bromine or hydrogen bromide must be prevented at the processing temperatures of the respective thermoplastic synthetic resin, since this causes corrosion damage to the processing devices. However, the thermal stability of the aromatic bromine compounds must not be so great that, when the synthetic resin wherein they are incorporated is exposed to a flame, there is no splitting off of bromine or hydrogen bromide, as is necessary for the flame-retarding effect. Furthermore, the flame retardant must not migrate from the synthetic resin to the surface of the latter, since it can be removed from that location by mechanical effects; in this way the concentration of flame retardant in the synthetic resin is reduced to below the limit of intended effectiveness. One example of an aromatic bromine compound, which should be especially suitable as a flame retardant for synthetic resins due to its high bromide content, is hexabromobenzene. However, hexabromobenzene migrates very rapidly to the surface of the synthetic resin; in addition, it sublimes even at temperatures lying in the range of the processing temperatures for the above-discussed thermoplastic synthetic resins.
Therefore, an increasing number of suggestions has been advanced in recent times for aromatic bromine compounds as flame retardants which are not utilized in the monomeric state but rather in the polymeric form. The idea is to retain, on the one hand, the high thermal stability and to maximally prevent the exuding or blooming out of the synthetic resin, on the other hand. Such compounds are, for example, polymeric pentabromobenzyl acrylate or methacrylate. It has been found, however, that in large-scale tests the production of a unitary polypentabromophenyl acrylate cannot be readily accomplished, so that the flame retardant which actually is satisfactory is very expensive in its manufacture.
This leaves the task of finding a flame retardant for thermoplastic synthetic resins which is simple to produce, does not split off bromine or hydrogen bromide at the processing temperatures of 220.degree.-280.degree. C. of these thermoplastic synthetic resins, and does not migrate from the synthetic resin.